Molecular Determinants for Ligand Binding at Serotonin 5-HT2A and 5-HT2C GPCRs: Experimental Affinity Results Analyzed by Molecular Modeling and Ligand Docking Studies

A comprehensive review of the ten main platelet receptors involved in platelet activity and cardiovascular disease

Cardiovascular disease (CVD) is a major cause of death worldwide. Although there are many variables that contribute to the development of this disease, it is predominantly the activity of platelets that provides the mechanisms by which this disease prevails. While there are numerous platelet receptors expressed on the surface of platelets, it is largely the consensus that there are 10 main platelet receptors that contribute to a majority of platelet function. Understanding these key platelet receptors is vitally important for patients suffering from myocardial infarction, CVD, and many other diseases that arise due to overactivation or mutations of these receptors. The goal of this manuscript is to review the main platelet receptors that contribute most to platelet activity.

A Novel Antibody Targeting the Second Extracellular Loop of the Serotonin 5-HT2A Receptor Inhibits Platelet Function

Serotonin (5-hydroxytriptamine or 5-HT) is known to be a weak platelet agonist, and is involved in thrombus formation. While 5-HT cannot induce platelet aggregation on its own, when secreted from the alpha granules, it binds to its G-protein Coupled Receptor (GPCR; i.e., 5HT_2AR), thereby acting to amplify platelet functional responses (e.g., aggregation). Thus, 5HT_2AR-mediated responses are more involved in the secondary amplification of platelet aggregation in the growing thrombus. Therefore, even though 5-HT can be seen as a weak inducer of platelet activation, it is an important amplifier of aggregation triggered by agonists such as ADP, collagen, and epinephrine, thereby enhancing thrombogenesis. The 5HT_2AR/5HT_2A signaling pathway is of clinical interest to the scientific and medical communities as it has been implicated in the genesis of several forms of cardiovascular disorders. However, efforts to develop antagonists for 5HT_2AR as therapeutic agents in cardiovascular diseases have thus far failed due to these reagents having deleterious side-effects, and/or to lack of selectivity, amongst other reasons. In light of research efforts that identified that the 5HT_2AR ligand binding domain resides in the second extracellular loop (EL2; amino acids P²⁰⁹-N²³³), we developed an antibody, i.e., referred to as 5HT_2ARAb, against the EL2 region, and characterized its pharmacological activity in the context of platelets. Thus, we utilized platelets from healthy human donors, as well as C57BL/6J mice (10-12 weeks old) to analyze the inhibitory effects of the 5HT_2ARAb on platelet activation in vitro, ex vivo, and on thrombogenesis in vivo as well as on 5HT_2AR ligand binding. Our results indicate that the 5HT_2ARAb inhibits 5-HT-enhanced platelet activation in vitro and ex vivo, but has no apparent effects on that which is agonist-induced. The 5HT_2ARAb was also found to prolong the thrombus occlusion time, and it did so without modulating the tail bleeding time, in mice unlike the P2Y12 antagonist clopidogrel and the 5HT_2AR antagonist ketanserin. Moreover, it was found that the 5HT_2ARAb does so by directly antagonizing the platelet 5HT_2AR. Our findings document that the custom-made 5HT_2ARAb exhibits platelet function blocking activity and protects against thrombogenesis without impairing normal hemostasis.

5-HT2C agonists and antagonists block different components of behavioral responses to potential, distal, and proximal threat in zebrafish

Serotonin (5-HT) receptors have been implicated in responses to aversive stimuli in mammals and fish, but its precise role is still unknown. Moreover, since at least seven families of 5-HT receptors exist in vertebrates, the role of specific receptors is still debated. Aversive stimuli can be classified as indicators of proximal, distal, or potential threat, initiating responses that are appropriate for each of these threat levels. Responses to potential threat usually involve cautious exploration and increased alertness, while responses to distal and proximal threat involve a fight-flight-freeze reaction. We exposed adult zebrafish to a conspecific alarm substance (CAS) and observed behavior during (distal threat) and after (potential threat) exposure, and treated with the 5-HT_2C receptor agonists MK-212 or WAY-161503 or with the antagonist RS-102221. The agonists blocked CAS-elicited defensive behavior (distal threat), but not post-exposure increases in defensive behavior (potential threat), suggesting inhibition of responses to distal threat. MK-212 blocked changes in freezing elicited by acute restraint stress, a model of proximal threat, while RS-102221 blocked changes in geotaxis elicited this stressor. We also found that RS-102221, a 5-HT_2C receptor antagonist, produced small effect on behavior during and after exposure to CAS. Preprint: https://www.biorxiv.org/content/10.1101/2020.10.04.324202; Data and scripts: https://github.com/lanec-unifesspa/5-HT-CAS/tree/master/data/5HT2C.

Importance of protein dynamics in the structure-based drug discovery of class A G protein-coupled receptors (GPCRs)

Demand for novel GPCR modulators is increasing as the association between the GPCR signaling pathway and numerous diseases such as cancers, psychological and metabolic disorders continues to be established. In silico structure-based drug design (SBDD) offers an outlet where researchers could exploit the accumulating structural information of GPCR to expedite the process of drug discovery. The coupling of structure-based approaches such as virtual screening and molecular docking with molecular dynamics and/or Monte Carlo simulation aids in reflecting the dynamics of proteins in nature into previously static docking studies, thus enhancing the accuracy of rationally designed ligands. This review will highlight recent computational strategies that incorporate protein flexibility into SBDD of GPCR-targeted ligands.

The adrenergic receptor antagonist carvedilol interacts with serotonin 2A receptors both in vitro and in vivo

There is increasing support for the potential clinical use of compounds that interact with serotonin 2A (5-HT_2A) receptors. It is therefore of interest to discover novel compounds that interact with 5-HT_2A receptors. In the present study, we used computational chemistry to identify critical ligand structural features of 5-HT_2A receptor binding and function. Query of compound databases using those ligand features revealed the adrenergic receptor antagonist carvedilol as a high priority match. As carvedilol is used clinically for cardiovascular diseases, we conducted experiments to assess whether it has any interactions with 5-HT_2A receptors. In vitro experiments demonstrated that carvedilol has high nanomolar affinity for 5-HT_2A receptors. In vivo experiments demonstrated that carvedilol increases the ethanol-induced loss of the righting reflex and suppresses operant responding in mice, and that these effects are attenuated by pretreatment with the selective 5-HT_2A receptor antagonist M100907. Moreover, carvedilol did not induce the head-twitch response in mice, suggesting a lack of psychedelic effects. However, carvedilol did not activate canonical 5-HT_2A receptor signaling pathways and antagonized serotonin-mediated signaling. It also reduced the head-twitch response induced by 2,5-Dimethoxy-4-iodoamphetamine, suggesting potential in vivo antagonism, allosteric modulation, or functional bias. These data suggest that carvedilol has functionally relevant interactions with 5-HT_2A receptors, providing a novel mechanism of action for a clinically used compound. However, our findings do not clearly delineate the precise mechanism of action of carvedilol at 5-HT_2A receptors, and additional experiments are needed to elucidate the role of 5-HT_2A receptors in the behavioral and clinical effects of carvedilol.

In Silico Studies Targeting G-protein Coupled Receptors for Drug Research Against Parkinson's Disease

Parkinson's Disease (PD) is a long-term neurodegenerative brain disorder that mainly affects the motor system. The causes are still unknown, and even though currently there is no cure, several therapeutic options are available to manage its symptoms. The development of novel antiparkinsonian agents and an understanding of their proper and optimal use are, indeed, highly demanding. For the last decades, L-3,4-DihydrOxyPhenylAlanine or levodopa (L-DOPA) has been the gold-standard therapy for the symptomatic treatment of motor dysfunctions associated to PD. However, the development of dyskinesias and motor fluctuations (wearing-off and on-off phenomena) associated with long-term L-DOPA replacement therapy have limited its antiparkinsonian efficacy. The investigation for non-dopaminergic therapies has been largely explored as an attempt to counteract the motor side effects associated with dopamine replacement therapy. Being one of the largest cell membrane protein families, G-Protein-Coupled Receptors (GPCRs) have become a relevant target for drug discovery focused on a wide range of therapeutic areas, including Central Nervous System (CNS) diseases. The modulation of specific GPCRs potentially implicated in PD, excluding dopamine receptors, may provide promising non-dopaminergic therapeutic alternatives for symptomatic treatment of PD. In this review, we focused on the impact of specific GPCR subclasses, including dopamine receptors, adenosine receptors, muscarinic acetylcholine receptors, metabotropic glutamate receptors, and 5-hydroxytryptamine receptors, on the pathophysiology of PD and the importance of structure- and ligand-based in silico approaches for the development of small molecules to target these receptors.

Mechanism Exploration of Arylpiperazine Derivatives Targeting the 5-HT2A Receptor by In Silico Methods

As a G-protein coupled receptor, the 5-hydroxytryptamine 2A (5-HT_2A) receptor is known for its critical role in the cognitive, behavioural and physiological functions, and thus is a primary molecular target to treat psychiatric diseases, including especially depression. With purpose to explore the structural traits affecting the inhibitory activity, currently a dataset of 109 arylpiperazine derivatives as promising 5-HT_2A antagonists was built, based on which the ligand-based three-dimensional quantitative structure-activity relationship (3D-QSAR) study by using both comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) approaches was carried out. The resultant optimal CoMSIA model displays proper validity and predictability with cross-validated correlation coefficient Q² = 0.587, non-cross-validated correlation coefficient R²_ncv = 0.900 and predicted correlation coefficient for the test set of compounds R²_pre = 0.897, respectively. Besides, molecular docking was also conducted to investigate the binding mode between these ligands and the active site of the 5-HT_2A receptor. Meanwhile, as a docking supplementary tool to study the antagonists' conformation in the binding cavity, molecular dynamics (MD) simulation was also performed, providing further elucidation about the changes in the ligand-receptor complex. Lastly, some new molecules were also newly-designed based on the above results that are potential arylpiperazine antagonists of 5-HT_2A receptor. We hope that the present models and derived information may be of help for facilitating the optimization and design of novel potent antagonists as antidepressant drugs as well as exploring the interaction mechanism of 5-HT_2A antagonists.

Similarities between the Binding Sites of SB-206553 at Serotonin Type 2 and Alpha7 Acetylcholine Nicotinic Receptors: Rationale for Its Polypharmacological Profile

Evidence from systems biology indicates that promiscuous drugs, i.e. those that act simultaneously at various protein targets, are clinically better in terms of efficacy, than those that act in a more selective fashion. This has generated a new trend in drug development called polypharmacology. However, the rational design of promiscuous compounds is a difficult task, particularly when the drugs are aimed to act at receptors with diverse structure, function and endogenous ligand. In the present work, using docking and molecular dynamics methodologies, we established the most probable binding sites of SB-206553, a drug originally described as a competitive antagonist of serotonin type 2B/2C metabotropic receptors (5-HT2B/2CRs) and more recently as a positive allosteric modulator of the ionotropic α7 nicotinic acetylcholine receptor (nAChR). To this end, we employed the crystal structures of the 5-HT2BR and acetylcholine binding protein as templates to build homology models of the 5-HT2CR and α7 nAChR, respectively. Then, using a statistical algorithm, the similarity between these binding sites was determined. Our analysis showed that the most plausible binding sites for SB-206553 at 5-HT2Rs and α7 nAChR are remarkably similar, both in size and chemical nature of the amino acid residues lining these pockets, thus providing a rationale to explain its affinity towards both receptor types. Finally, using a computational tool for multiple binding site alignment, we determined a consensus binding site, which should be useful for the rational design of novel compounds acting simultaneously at these two types of highly different protein targets.

On the Basicity of 8-Phenylsulfanyl Quipazine Derivatives: New Potential Serotonergic Agents

A protonation state of serotonergic ligands plays a crucial role in their pharmacological activity. In this research, the basicity of 8-phenylsulfanyl quipazine derivatives as new potential serotonergic agents was studied. The most favorable protonation sites were determined in the gas and aqueous phases. In water, a solvation effect promoting the protonation of the N3 atom overcomes a positive charge delocalization phenomenon favoring a N1 atom protonation. The most stable conformations of neutral and protonated molecules in gas and water were found. It was demonstrated that a diprotonation reaction may occur. The most favorable among the diprotonated structures is the molecule with the N1 and N3 atoms protonated. A calculation of the pKa and pKa2 in water of a set of monosubstituted 8-phenylsulfanyl quipazine derivatives was performed using B3LYP/6-31G(d) and the SMD continuum solvation model. Enthalpic and entropic contributions to the pKa and pKa2 in gas and water were separated for a rationalization of a substituent effect on values of the pKa and pKa2. The relationship of the proton affinity and the solvation enthalpy in water with some reactivity descriptors, such as the Fukui function, the molecular electrostatic potential (MEP), and the global softness, was investigated. The order of the pKa values is the most controlled by the entropy. The diprotonation reaction, despite having an unfavorable enthalpy in water, is driven entropically. Final state effects in the diprotonated species were analyzed with the triadic formula. Results of a calculation of the theoretical basicity of the 8-phenylsulfanyl quipazines indicate that they should be monoprotonated on the N3 atom in the CNS environment. Diprotonation of the studied compounds may occur in very acidic body fluids such as the gastric juice.

Novel 4-substituted-N,N-dimethyltetrahydronaphthalen-2-amines: synthesis, affinity, and in silico docking studies at serotonin 5-HT2-type and histamine H1 G protein-coupled receptors

Syntheses were undertaken of derivatives of (2S,4R)-(-)-trans-4-phenyl-N,N-dimethyl-1,2,3,4-tetrahydronaphthalen-2-amine (4-phenyl-2-dimethylaminotetralin, PAT), a stereospecific agonist at the serotonin 5-HT2C G protein-coupled receptor (GPCR), with inverse agonist activity at 5-HT2A and 5-HT2B GPCRs. Molecular changes were made at the PAT C(4)-position, while preserving N,N-dimethyl substitution at the 2-position as well as trans-stereochemistry, structural features previously shown to be optimal for 5-HT2 binding. Affinities of analogs were determined at recombinant human 5-HT2 GPCRs in comparison to the phylogenetically closely-related histamine H1 GPCR, and in silico ligand docking studies were conducted at receptor molecular models to help interpret pharmacological results and guide future ligand design. In most cases, C(4)-substituted PAT analogs exhibited the same stereoselectivity ([-]-trans>[+]-trans) as the parent PAT across 5-HT2 and H1 GPCRs, albeit, with variable receptor selectivity. 4-(4'-substituted)-PAT analogs, however, demonstrated reversed stereoselectivity ([2S,4R]-[+]-trans>[2S,4R]-[-]-trans), with absolute configuration confirmed by single X-ray crystallographic data for the 4-(4'-Cl)-PAT analog. Pharmacological affinity results and computational results herein support further PAT drug development studies and provide a basis for predicting and interpreting translational results, including, for (+)-trans-4-(4'-Cl)-PAT and (-)-trans-4-(3'-Br)-PAT that were previously shown to be more potent and efficacious than their corresponding enantiomers in rodent models of psychoses, psychostimulant-induced behaviors, and compulsive feeding ('binge-eating').

Molecular modeling of 5HT2A receptor - arylpiperazine ligands interactions

In this paper, we report the molecular modeling of the 5HT2A receptor and the molecular docking of arylpiperazine-like ligands. The focus of the research was on explaining the effects the ligand structure has on the binding properties of the 5HT2A receptor and on the key interactions between the ligands and the receptor-binding site. To see what the receptor–ligand interactions were, various substituents were introduced in one part of the ligand, keeping the rest unchanged. In this way, using a docking analysis on the proposed 5HT2A receptor model, we identified key receptor–ligand interactions and determined their properties. Those properties were correlated with experimentally determined binding affinities in order to determine the structure to activity relationship of the examined compounds.

Insights into the influence of 5-HT2c aminoacidic variants with the inhibitory action of serotonin inverse agonists and antagonists

Specific modulation of serotonin 5-HT(2C) G protein-coupled receptors may be therapeutic for obesity and neuropsychiatric disorders. The different efficacy of drugs targeting these receptors are due to the presence of genetic variants in population and this variability is still hard to predict. Therefore, in order to administer the more suitable drug, taking into account patient genotype, it is necessary to know the molecular effects of its gene nucleotide variations. In this work, starting from an accurate 3D model of 5-HT(2C), we focus on the prediction of the possible effect of some single nucleotide polymorphisms (SNPs) producing amino acidic changes in proximity of the 5-HT(2C) ligand binding site. Particularly we chose a set of 5-HT(2C) inverse agonists and antagonists which have high inhibitory activity. After prediction of the structures of the receptor-ligand complexes using molecular docking tools, we performed full atom molecular dynamics simulations in explicit lipid bilayer monitoring the interactions between ligands and trans-membrane helices of the receptor, trying to infer relations with their biological activity. Serotonin, as the natural ligand was chosen as reference compound to advance a hypothesis able to explain the receptor inhibition mechanism. Indeed we observed a different behavior between the antagonists and inverse agonist with respect to serotonin or unbounded receptor, which could be responsible, even if not directly, of receptor's inactivation. Furthermore, we analyzed five aminoacidic variants of 5HT(2C) receptor observing alterations in the interactions between ligands and receptor which give rise to changes of free energy values for every complex considered.

Aromatic interactions impact ligand binding and function at serotonin 5-HT2C G protein-coupled receptors: Receptor homology modeling, ligand docking, and molecular dynamics results validated by experimental studies

The serotonin (5-hydroxytryptamine, 5-HT) 5-HT2 G protein-coupled receptor (GPCR) family consists of types 2A, 2B, and 2C that share ~75% transmembrane (TM) sequence identity. Agonists for 5-HT2C receptors are under development for psychoses, whereas, at 5-HT2A receptors, antipsychotic effects are associated with antagonists-in fact, 5-HT2A agonists can cause hallucinations and 5-HT2B agonists cause cardiotoxicity. It is known that 5-HT2A TM6 residues W6.48, F6.51, and F6.52 impact ligand binding and function, however, ligand interactions with these residues at the 5-HT2C receptor has not been reported. To predict and validate molecular determinants for 5-HT2C-specific activation, results from receptor homology modeling, ligand docking, and molecular dynamics (MD) simulation studies were compared with experimental results for ligand binding and function at wild type and W6.48A, F6.51A, and F6.52A point-mutated 5-HT2C receptors.

Molecular pharmacology and ligand docking studies reveal a single amino acid difference between mouse and human serotonin 5-HT2A receptors that impacts behavioral translation of novel 4-phenyl-2-dimethylaminotetralin ligands

During translational studies to develop 4-phenyl-2-dimethylaminotetralin (PAT) compounds for neuropsychiatric disorders, the (2R,4S)-trans-(+)- and (2S,4R)-trans-(-)-enantiomers of the analog 6-hydroxy-7-chloro-PAT (6-OH-7-Cl-PAT) demonstrated unusual pharmacology at serotonin (5-HT) 5-HT2 G protein-coupled receptors (GPCRs). The enantiomers had similar affinities (Ki) at human (h) 5-HT2A receptors (≈ 70 nM). In an in vivo mouse model of 5-HT2A receptor activation [(±)-(2,5)-dimethoxy-4-iodoamphetamine (DOI)-elicited head twitch], however, (-)-6-OH-7-Cl-PAT was about 5-fold more potent than the (+)-enantiomer at attenuating the DOI-elicited response. It was discovered that (+)-6-OH-7-Cl-PAT (only) had ≈ 40-fold-lower affinity at mouse (m) compared with h5-HT2A receptors. Molecular modeling and computational ligand docking studies indicated that the 6-OH moiety of (+)- but not (-)-6-OH-7-Cl-PAT could form a hydrogen bond with serine residue 5.46 of the h5-HT2A receptor. The m5-HT2A as well as m5-HT2B, h5-HT2B, m5-HT2C, and h5-HT2C receptors have alanine at position 5.46, obviating this interaction; (+)-6-OH-7-Cl-PAT also showed ≈ 50-fold lower affinity than (-)-6-OH-7-Cl-PAT at m5-HT2C and h5-HT2C receptors. Mutagenesis studies confirmed that 5-HT2A S5.46 is critical for (+)- but not (-)-6-OH-7-Cl-PAT binding, as well as function. The (+)-6-OH-7-Cl-PAT enantiomer showed partial agonist effects at h5-HT2A wild-type (WT) and m5-HT2A A5.46S point-mutated receptors but did not activate m5-HT2A WT and h5-HT2A S5.46A point-mutated receptors, or h5-HT2B, h5-HT2C, and m5-HT2C receptors; (-)-6-OH-7-Cl-PAT did not activate any of the 5-HT2 receptors. Experiments also included the (2R,4S)-trans-(+)- and (2S,4R)-trans-(-)-enantiomers of 6-methoxy-7-chloro-PAT to validate hydrogen bonding interactions proposed for the corresponding 6-OH analogs. Results indicate that PAT ligand three-dimensional structure impacts target receptor binding and translational outcomes, supporting the hypothesis that GPCR ligand structure governs orthosteric binding pocket molecular determinants and resulting pharmacology.

Molecular and behavioral pharmacology of two novel orally-active 5HT2 modulators: potential utility as antipsychotic medications

BACKGROUND

Desired serotonin 5HT2 receptor pharmacology for treatment of psychoses is 5HT2A antagonism and/or 5HT2C agonism. No selective 5HT2A antagonist has been approved for psychosis and the only approved 5HT2C agonist (for obesity) also activates 5HT2A and 5HT2B receptors, which can lead to clinical complications. Studies herein tested the hypothesis that a dual-function 5HT2A antagonist/5HT2C agonist that does not activate 5HT2B receptors would be suitable for development as an antipsychotic drug, without liability for weight gain.

METHODS

The novel compounds (+)- and (-)-trans-4-(4'-chlorophenyl)-N,N-dimethyl-2-aminotetralin (p-Cl-PAT) were synthesized, characterized in vitro for affinity and functional activity at human 5HT2 receptors, and administered by intraperitoneal (i.p.) and oral (gavage) routes to mice in behavioral paradigms that assessed antipsychotic efficacy and effects on feeding behavior.

RESULTS

(+)- and (-)-p-Cl-PAT activated 5HT2C receptors, with (+)-p-Cl-PAT being 12-times more potent, consistent with its higher affinity across 5HT2 receptors. Neither p-Cl-PAT enantiomer activated 5HT2A or 5HT2B receptors at concentrations up to 300-times greater than their respective affinity (Ki), and (+)-p-Cl-PAT was shown to be a 5HT2A competitive antagonist. When administered i.p. or orally, (+)- and (-)-p-Cl-PAT attenuated the head-twitch response (HTR) in mice elicited by the 5HT2 agonist (-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and reduced intake of a highly palatable food in non-food-deprived mice, with (+)-p-Cl-PAT being more potent across behavioral assays.

CONCLUSIONS

The novel in vitro pharmacology of (+)-p-Cl-PAT (5HT2A antagonism/5HT2C agonism without activation of 5HT2B) translated in vivo to an orally-active drug candidate with preclinical efficacy to treat psychoses without liability for weight gain.